Recently, along with an increase in the speed of image forming apparatuses, a configuration in which a plurality of image forming portions having developers of different colors are arranged side by side and the image forming processes of respective colors are processed in parallel is mainstream.
For example, in a full-color image forming apparatus in an electrophotographic type, there is an intermediate transfer tandem type in which toner images of the respective colors are sequentially overlapped and transferred onto a surface of an intermediate transfer belt by a primary transfer roller (transfer member), and after that, a full-color toner image is collectively transferred onto a recording material. This intermediate transfer tandem type has advantages that can cope with high productivity or conveyance of various media. The intermediate transfer belt is driven to travel in a state of being stretched by a plurality of stretching rollers including a driving roller.
In such an intermediate tandem type image forming apparatus, in order to reduce the abutting pressure of the primary transfer roller, a configuration in which the primary transfer roller is offset from a photosensitive member toward the downstream side in a conveyance direction of the intermediate transfer belt has been proposed (see Patent Literature 1).
In an intermediate transfer tandem type image forming apparatus, in the case of forming an image of a monochrome single color, a configuration in which a primary transfer roller included in a color image forming portion is separated from an intermediate transfer belt has been known.
Here, the tension of the intermediate transfer belt may change according to the change in the stretching layout of the intermediate transfer belt at the time of monochrome image formation and at the time of color image formation. In this case, if the primary transfer rollers are arranged as disclosed in Patent Literature 1, the force received by a black primary transfer roller which is not separated from the intermediate transfer belt may change and the following phenomenon may occur.
That is, along with the change from the stretching layout at the time of color image formation to the stretching layout at the time of monochrome image formation, when the tension of the intermediate transfer belt changes from a high state to a low state, the pressing force of the intermediate transfer belt to the primary transfer roller is weakened. The primary transfer roller 55 is applied with a biasing force by a primary transfer spring. Therefore, when the pressing force of the intermediate transfer belt 57 against the biasing force decreases, the primary transfer roller 55 changes from a position indicated by a solid line to a position indicated by a dashed line as illustrated in FIG. 7. In addition, as the position of the primary transfer roller 55 changes, a contact point P2 between the primary transfer roller 55 and the intermediate transfer belt 57 changes to a contact point P3. Therefore, a contact point distance from a contact point P1 between a photosensitive drum 51 and the intermediate transfer belt 57 to a contact point between the primary transfer roller 55 and the intermediate transfer belt 57 changes from L1 to L2. Along with the change of the contact point distance, a primary transfer current flowing from the primary transfer roller 55 to the photosensitive drum 51 changes when a bias is applied to the primary transfer roller 55, because the intermediate transfer belt 57 is a resistor. This may cause image defects.
Accordingly, the present invention has been made in view of such circumstances, and an object of the present invention is to provide an intermediate transfer unit capable of suppressing a change in a current flowing from a transfer member to a photosensitive member in a configuration in which a part of a plurality of transfer members can be separated from an intermediate transfer belt.